1. Field of Invention
This invention relates to apparatus for the development of exposed photographic material, and in particular to apparatus for developing lithographic offset printing plates.
2. Description of the Prior Art
In an apparatus or system for the development of aluminium lithographic printing plates of the type disclosed in EP-A-410500 it is advantageous to maintain the strength of the processing liquids within specified limits so as to achieve consistent high quality printing plates, and reduce waste products.
This can be achieved by constantly topping up a process liquid reservoir or bath by automatic means as photographic material is processed and the both chemicals are consumed.
The exhaustion of the chemicals in the baths can normally be directly correlated with the amount of processed material passing through the apparatus. This in turn is directly related to the surface area of the material being processed.
A known surface area detection means comprises one sensor for sensing the length of processed material, and an array of sensors to determine the width of the material. Such a system is relatively expensive since to measure random material widths with precision requires a large number of sensors. For example, offset printing plates may vary in width from 225 mm to 800 mm and if it is necessary to measure all possible combinations of standard material widths from 200 mm upwards within a 10% accuracy range some 30 sensors are necessary, and within a 20% accuracy range some 16 sensors are necessary.
Another known surface area detection means is disclosed in JP-A-4 166 935 (Fuji Photo Film Co. Ltd.). This system likewise comprises one sensor for sensing the length of processed material. The width size of the material is determined on the basis of a previously set ratio of length to width in the processing direction of the photosensitive material. It is clear that this system will work satisfactorily only if all the different sheet formats being processed have (nearly) the same ratio of length to width, and if all the sheets are being processed lengthwise. On the contrary, if a sheet is being processed widthwise, the length sensor will errorneously consider a smaller dimension, in accordance with the set ratio, as being the width belonging to the measured "length". Also, the use of the ratio length to width for the control of the replenishment forms a serious limitation of the number of sheet formats that can be used in the apparatus.